It is known to use low ohmage shunt resistors to measure current through a load. The resistor is connected in series with the load such that the voltage across the resistor represents load current. This voltage can be used as an input to a detector circuit to determine when measured current rises above or falls below some decision point value.
A problem arises where it is essential to detect and respond to current changes very quickly. Resistors of traditional construction have both body and lead inductance, a characteristic which generates voltage due to a signal with a high di/dt value. For example, a ten-milliohm resistor may have about 4nH inductance. If current drops from 16 amps to 0 within a 75 nanosecond switch-off time, the inductance-induced spike voltage is 850 millivolts, more than 5 times larger than the useful voltage drop across the shunt resistor. Therefore, it may be impossible to detect and respond to rapid current changes using control circuits with conventional shunt resistors.
The problem described above becomes especially acute if pulse width modulation is used to control the magnitude of the load current. PWM techniques are characterized by frequent and rapid transitions in current; i.e., high di/dt factors.